REDWOOD CITY -- A ravenous six-alarm fire swept through a Redwood City apartment building and sent residents scrambling to safety by climbing down balconies and, in one astonishing sequence, rappelling four stories to the ground.

Only four people were injured, none seriously, in the blaze that shot bright orange flames into the early Thursday sky and left officials lamenting the absence of fire sprinklers for why it burned so wildly and quickly out of control.

"I guarantee if this building had been sprinklered this wouldn't have happened," Redwood City Fire Marshal Jim Palisi said as crews narrowed in on controlling the blaze in mid-morning.

The building was not required to have sprinklers because it was built before 1989, when state law made them mandatory for new construction at least three stories tall or containing at least 16 housing units.

According to the Redwood City Fire Department officials, the fire started at 5:17 a.m. in a fourth-floor unit and spread to the third, damaging 20 units in all.

Fire and smoke quickly filled an interior hallway on the top floor, blocking the exit path for several residents and forcing them to run to balconies where firefighters from Redwood City and throughout San Mateo County helped them down with ground ladders.

Within 45 minutes, the roof collapsed.

As Oskar Lizarraga-Davis, girlfriend Amy Purtle and their 6-month-old son Oscar Jr. fled from their third-floor apartment, they recalled seeing their upstairs neighbors make a precarious climb down to their balcony. Lizarraga-Davis, 30, said he even saw two residents who happen to be rock climbers rappel to the ground from their top-floor unit.

The American Red Cross was summoned and set up an evacuation center at the Red Morton Community Center on Roosevelt Avenue. A stream of residents showed up throughout the morning for news, basic amenities or help finding housing. About 75 people sought help at the shelter.

Thursday evening, the Red Cross announced an overnight shelter was opened at the Senior Wellness Center, 1455 Madison Ave., Redwood City. A center to provide needs assessment and recovery planning for affected residents was set to open at the center Friday through Sunday, noon to 7 p.m.

The fire also forced the rush-hour closure of Woodside -- doubling as Highway 84 in the area -- for a roughly mile-long stretch between Gordon Street to the north and Massachusetts Avenue to the south. The closure lasted throughout the day.

A cause for the fire remains under investigation, and it was unclear how extensively it damaged the property, which was last assessed at a value approaching $11 million.

The blaze evoked memories of a July 7 six-alarm fire about three miles south, also on Woodside Road, where a man died in an apartment complex that lacked fire sprinklers because it was similarly grandfathered from the law.

Sprinklers will be mandatory if the owner of the apartments in Thursday's fire decides to rebuild. Otherwise, the fire marshal said, "there's no retroactive requirement to make people sprinkler their building." The owner of the four-story, 73-unit complex at 926 Woodside Road -- a ground-level parking garage topped by three residential floors -- could not be reached for comment.

Palisi, the fire marshal, said the building is not habitable and he had no estimate for when, or even whether, residents can return.

Second-floor resident Noy Savanh, 35, was showering and getting ready for work Thursday in Palo Alto when he heard fire alarms sounding.

He quickly roused his wife, 2-year-old daughter and an 18-year-old cousin visiting from Thailand, and they fled with the clothes on their backs.

"When we came outside, we saw the fire was burning," Savanh said.

On Thursday morning, he wondered aloud if such mementos as his wife's wedding necklace and family photos survived.

Palisi said when firefighters arrived they quickly upgraded the fire response to two, three and eventually a rare sixth alarm. Firefighters went into "a rescue operation immediately." Of the four people injured, two of the injuries were considered moderate and the other two were deemed minor.

Oksana Ushkalova, 31, a resident of the complex where the fire happened, awoke to a commotion and heard someone scream, "Fire! Get out!" The Ukranian-born marketing director is worried about the passport and childhood photos she left inside her second-floor unit, and found herself thinking about how quickly her life was upended.

"I went to sleep and everything was fine," Ushkalova said. "It's sort of dawning on me in stages. I'm alive, that's a good thing."

Staff writer Joshua Melvin contributed to this report.

RED CROSS ASSISTANCE

IF YOU NEED HELPAn overnight shelter is now open to those affected by the fire. The Overnight Shelter is located at the Senior Wellness Center, 1455 Madison Avenue in Redwood City.

A Client Services Center will provide affected residents with needs assessment and recovery planning from Friday through Sunday, noon to 7 p.m. at the Senior Wellness Center. The Evacuation Center, which opened this morning at the Red Morton Community Center, closed at 8 p.m. Thursday.

HOW PEOPLE CAN HELP People can make financial donations to support local Red Cross disaster relief by visiting www.redcross.org or calling 1-800-RED-CROSS or texting REDCROSS to 90999 to make a $10 donation. Contributions may also be sent to the Bay Area Chapter, 1663 Market Street, San Francisco, CA 94103, to your local Red Cross chapter, or to the national office, American Red Cross, P.O. Box 37243, Washington, DC 20013.

IN KIND DONATIONS The Red Cross appreciates the good intentions of people wanting to donate items to those affected in Redwood City. In-kind donations are not needed and cannot be accepted at the shelter. Contact partner agency, the Redwood City Salvation Army, at (650) 368-4643, about any in-kind donation questions.